Data storage systems often utilize several storage devices such as hard magnetic or optical disk drives to provide adequate permanent data storage capacity. Each hard disk drive incorporates a multiple hard magnetic or optical disk stack mounted on a spindle and driven by a motor operated at a predetermined rotational speed. Data transfer to and from each disk drive is effected by means of a plurality of read/write transducers, such as magnetic read/write heads, each one of which is associated with a disk surface and is mounted on a common actuator mechanism to enable transmission of data as well as accurate position information to and from the associated disk surface.
To ensure acceptable data access time and reliable read/write head positioning performance of the disk drive, disk spindle rotation must be accurately controlled. This objective is met by means of a phase locked loop servomechanism provided for each disk drive. The phase locked loop servomechanism operates to follow a periodic rotation reference signal provided by a command signal source. In the traditional implementation of the phase locked loop servomechanism both the frequency and the phase of a feedback signal and the periodic rotation reference signal are compared to generate an error which is regulated to an acceptably low level. Other implementations of the phase locked loop servomechanism rely only on frequency equalization or lock between the feedback signal and the periodic rotation reference signal. The frequency of the rotation reference signal corresponds to the desired rotational speed of the disk drive spindle.
With the spindle of each disk drive in the data storage system following its own rotation reference signal, the position of the currently selected read/write head relative to its disk surface at any moment of time varies from one disk drive to another. Disk surfaces of different disk drives can, therefore, be accessed only in a sequential manner and throughput is limited by the random spatial relationship between the selected read/write head and the desired location on the associated disk surface.
Significant increase in data throughput of a multiple disk drive storage system can be achieved by synchronizing the rotational motion of the spindles of a number of identical disk drives. In this arrangement the disk spindle of every disk drive in the data storage system is driven by a common reference signal. Consequently, individual disk drive spindles are accelerated and maintained at the same rotational speed. At all times, the currently enabled read/write head of each disk drive is spatially aligned relative to its disk surface in exactly the same manner as the read/write heads of all the other disk drives. This permits highly parallel data transfer operation to and from a number of the disk drives of the multiple disk drive storage system by utilizing an appropriate data storage format.
A known way of establishing identical angular alignment of the disk spindles of several disk drives with respect to the common reference signal relies on the use in each disk drive of a special disk surface and an associated read/write head which are dedicated to encoding and relaying position information for closed loop control of both the read/write head actuator and the disk spindle. A frame of reference for defining rotation of the disk spindle is given by an index signal written on the dedicated disk surface and detected once for every revolution of the disk spindle. Such an index signal can be easily recorded on every track along an appropriate radially coherent or continuous curve described by the dedicated read/write head as it is made to move across the dedicated disk surface. The radial index curve will generally take the form of a circular arc when the read/write head motion is obtained by means of a rotary actuator. Conversely, whenever the linear read/write head actuator mechanism is used, the radial index curve will be disposed along a radius on the dedicated disk surface. In any event, the resultant index signal, for example in the form of a pulse, is periodic and can be directly compared to an externally provided command signal waveform. The phase locked loop servomechanism determines the phase misalignment between the index signal and the external command signal and acts to reduce this error until the disk spindle attains the desired rotational speed and position orientation with respect to its index signal. At this time the phase locked loop servomechanism of the disk spindle is said to have acquired lock and tracks the command signal within an acceptable steady state phase error.
Phase alignment of the identical index signals, obtained from the dedicated surface of each disk drive, to a common command signal is a convenient way of synchronizing the rotational motion of the disk spindles from different disk drives. As new requests to transfer data are processed, one disk surface from each disk drive can be selected for immediate access simultaneously.
A difficulty of employing such disk surface recorded position information for disk spindle control arises with disk surface data formats that preclude the use of a disk surface and read/write head dedicated to position information processing. Instead, such position information is encoded on all disk surfaces of each disk drive along with the data. To utilize the data storage capacity of a disk drive efficiently, the position information may be embedded along the boundaries of circumferentially organized continuous sectors containing blocks of data. Data and position information can be placed on a disk surface in a variety of formats according, inter alia, to the disk drive performance characteristics. Since the position information is obtained from the currently selected disk surface, each surface of the disk drive must contain its own index signal for disk spindle control.
As the disk drive receives a request to access data located on another of its disk surfaces, a switch to the new read/write head is made and the phase locked loop servomechanism discontinues processing of the index signal from the previously selected disk surface and initiates detection of the new index signal on the newly accessed disk surface. Any angular misalignment between the two index signals which may be desirable, for example, to enhance the speed of formatting the disk surfaces, creates a phase error between the command signal and the feedback index signal which is acted upon by the phase locked loop servomechanism to create a corrective action. Consequently, data transfer is delayed until the process of re-acquiring lock with respect to the new index signal, occurring over a period of time dependent upon the dynamic characteristics of the phase locked loop servomechanism, is complete.
Another disadvantage of embedding the index information on the disk surface at the boundaries of the data blocks is the constraint imposed on the choice of data formats. Since such embedded index signal must be located along a radially coherent or continuous curve on each disk surface of the disk drive, any acceptable data format must ensure that the data area boundaries align along that curve. However, to ensure uniform aerial bit density throughout the disk surface, it may be desirable to have a greater number of data areas or sectors on outward tracks than on the tracks proximal to the center of the disk surface. To this end, bands of radially contiguous tracks can be grouped together to conform to a distinct sector format making sector alignment along any one radially coherent or continuous curve on the disk surface highly undesirable.